The mutagenic activity of ethylmethane sulfonate (EMS) as a function of its DNA alkylating ability has been studied in Salmonella typhimurium. The mutagenic activity of EMS in the base-pair substitution strain G-46 and its repair deficient derivatives (TA1950, [uvrB]; TA92, [pKM101]; TA2410, [uvrB, pKM101]) were compared. Mutation frequencies increased as a function of total DNA alkylation. Ethylation levels were equivalent in wild type and uvrB- cells, but the efficiency of induction of mutations (mutants/adduct) was different between the two cell types. This will provide a reference for the effects of the various repair deficiencies on EMS-induced mutagenesis and provide data to allow us to relate treated dose to delivered dose to mutagenic response.